Polar

The surface energy/tension of a substance is due to different interactions. A distinction is made between disperse and polar interactions. The surface energy of a substance is composed of a disperse and a polar component. Polar interactions are strong and long-range interactions between molecules that are based on electrostatic attraction. In many molecules, the charge is not evenly distributed, but is present in the form of partial charges. Unequal charges attract each other, resulting in strong interactions between the charges.

A distinction is made in polar interactions between different contributions that are of different strengths and have different ranges. The strongest interactions are the interactions between ions. With a distance dependence of 1/r, these are also very long-range. Then come the interactions between a permanent dipole and ions with a distance dependence of 1/r². The dipole-dipole interactions are much weaker. Here, the distance dependence depends on whether the dipoles can rotate freely or not. A special case of the dipole-dipole interactions are the hydrogen bonds. These are very targeted interactions that act between hydrogen atoms and negative partial charges. These strong interactions are the reason for the high surface tension of water. If there are no permanent charges, they can be induced. These disperse forces are much weaker than the polar ones, but they play an important role e.g. in plastics.

While disperse interactions are also possible without permanent dipoles, charges are needed for polar interactions. In particular, so-called low-energy surfaces with a low surface energy and a low polar fraction often cannot be further processed without pre-treatment. One possibility of pre-treatment is plasma treatment, in which reactive species remove the finest impurities from the surface and additionally functionalise the surface through the addition of polar groups. The polar groups contribute to the polar interactions with their dipole moment, which is also reflected in the polar fraction of the surface energy. In many applications, it has been shown that only a sufficiently large polar fraction enables wetting of surfaces with many lacquers, adhesives and other substances.

There can be no substances whose surface tension/energy is only polar, since disperse interactions occur in all atoms and molecules. However, there are substances whose surface energy/voltage is purely disperse because they have no polar groups.

By comparing the disperse and polar fractions of two phases, predictions can be derived about their adhesion to each other. The more the disperse and polar fractions match, the more interaction possibilities there are between the phases. The stronger the adhesion can then be expected (see figures 1 and 2). A low interfacial tension/energy is shown with a high interaction potential between two phases.